Ionic Conductivity in the Solid Glyme Complexes[CH3O(CH2CH2O)nCH3]:LiAsF 6 (n ) 3,4)

Chuhong Zhang, David Ainsworth, Yuri G. Andreev, and Peter G. Bruce*

EastChem, School of Chemistry, UniVersity of St. Andrews, St. Andrews, Fife KY16 9ST, Scotland

Received May 4, 2007; E-mail: p.g.bruce@st-and.ac.uk

Ionic conductivity in the solid state has traditionally beendominated by ceramic materials (e.g., the O2- ion conductor[Zr1-2xY2x]O2-x or the Na+ conductor NaâAl2O3) and polymericmaterials (e.g., PEOx:LiN(SO2CF3)2 or PEO6:LiAsF6).1,2 Severalyears ago ionic conductivity was also recognized in plasticcrystalline solids.3 Very recently, we presented the first report ofionic conductivity in a new class of solid electrolytes.4 Such solidsconsist of cations coordinated by discrete, low molecular weight,ligands, such as the glyme [CH3O(CH2CH2O)3CH3]. They aredistinct from ceramic electrolytes, being soft solids, and frompolymer electrolytes, which consist of molecules of higher (greaterthan 1000 Da) and distributed molecular weight. Whereas the crystalstructure of a polymer electrolyte remains the same over a widerange of molecular weights (103-106 Da), low molecular weightcomplexes exhibit a rich variety of structures on varying themolecular weight of their ligands.5 Here we present results on twolow molecular weight complexes, [CH3O(CH2CH2O)3CH3]:LiAsF6

and [CH3O(CH2CH2O)4CH3]:LiAsF6, hereafter designated as G3:LiAsF6 and G4:LiAsF6. Despite differing by only one ethylene oxide(EO) unit, their crystal structures are very different (in contrast topolymer electrolytes). We show that this difference leads tomarkedly varying cation transport numbers (t+ ) 0.8 for G3:LiAsF6

and 0.1 for G4:LiAsF6), yet their conductivities are remarkablysimilar. Learning how to construct solid electrolytes that exhibithigh Li+ transport numbers is important in minimizing polarizationwhen such electrolytes are used in all-solid-state cells, such aslithium batteries.

Powder X-ray diffraction patterns for G3:LiAsF6 and G4:LiAsF6complexes are in good agreement with those calculated fromprevious single-crystal data, as illustrated for the G3:LiAsF6

complex in Figure 1.6

Conductivity measurements were carried out using ac impedancespectroscopy on pressed pellets between blocking (stainless steel)electrodes. A representative complex impedance plot is availablein the Supporting Information. In all cases, a single semicircle witha lower frequency spike was observed. The semicircle wasassociated with a capacitance of 1-2 pFcm-1, indicating that theimpedance is dominated by bulk (intracrystalline) response, withno significant contribution from grain boundary resistances.

Conductivities at different temperatures were extracted from theac impedance plots and are presented in Figure 2a (for G3:LiAsF6

and G4:LiAsF6).The linear variation of logσ versus 1/T for both complexes is

consistent with ion hopping between fixed sites, rather than thecurved plots normally associated with conduction in amorphouspolymer electrolytes. The conductivities of the two glymes reportedin Figure 2a are similar, something that, at first sight, might bethought consistent with the difference of only one EO unit betweenthe G3 and G4 complexes. However, the crystal structures of thetwo complexes are very different, and this is reflected in thedifference in the activation energies for G3:LiAsF6 (55 kJ mol-1)

and G4:LiAsF6(68 kJ mol-1). These results encouraged a deeperanalysis of ion transport in the two glyme complexes, involvingdetermination of their transport numbers. The method of combinedac and dc measurements, described previously by Bruce and

Figure 1. Experimental (bottom) and calculated (top) powder X-raydiffraction patterns for G3:LiAsF6.

Figure 2. (a) Ionic conductivity,σ, as a function of temperature. (b) Thedc current as a function of time duringt+ measurements.

Published on Web 06/21/2007

8700 9 J. AM. CHEM. SOC. 2007 , 129, 8700-8701 10.1021/ja073145f CCC: $37.00 © 2007 American Chemical Society

Vincent, was employed to determine the transport numbers of thetwo solid complexes.7 The variation of the dc currents as a functionof time for the G3 and G4 complexes is presented in Figure 2b.The cation transport numbers,t+, are 0.8 (G3:LiAsF6) and 0.1 (G4:LiAsF6). The transport number measurements were made at 50°Cbecause the lower conductivities at lower temperatures precludedaccurate measurements. Unlike the conductivities, which at 50°Cdiffer by only a factor of 1.3, thet+ values differ by a factor of 8.

A rationale for the very different transport numbers between thesetwo glymes, differing by only 1 EO unit, may be found bycomparing their crystal structures (Figures 3 and 4).

In the case of the G3:LiAsF6 complex, the G3 molecules arearranged in rows parallel to thea-axis. In each row, the moleculesform a tunnel within which the Li+ ions reside. A G3 moleculecontains four ether oxygens. Each Li+ ion is coordinated by twoether oxygens from each of two neighboring glyme molecules(Figure 3, bottom). The Li+ ion is five coordinated, the fifth positionbeing occupied by a fluorine from an AsF6

- anion. Each glymemolecule bridges between two neighboring Li+ ions. The anionsare also arranged in rows, with each anion coordinating to onlyone Li+ ion, its remaining interactions being with the surroundingglyme molecules. Although this structure is different from that of

the PEO6:LiXF6 (X ) P, As, or Sb), there are similarities in thatthe 6:1 polymer electrolyte consists of PEO tunnels lined by etheroxygens and within which the Li+ ions reside. The presence oftunnels for Li+ ion transport in both G3 and PEO crystallinestructures promotes a hight+ in both cases.2 Comparing the G3with the G4 structure (Figure 4), the G4 complex lacks continuouspathways for Li+ ion transport. Pairs of Li+ ions and pairs of G4molecules form discrete binuclear complexes, where each Li+ ionis coordinated by three ether oxygens from one glyme and by twofrom the other. Each ether oxygen coordinates to only one Li ion,that is, there are two Li+ ions and 10 ether oxygens in each binuclearcomplex. There are no continuous pathways from one complex tothe next in any direction since these are interrupted by the presenceof AsF6

- anions and are too far apart (Figure 4).Considering the anion transport in G3 and G4 complexes, in the

latter case, the anions only interact with the glyme molecules, suchinteractions are of a weak van der Waals nature. In contrast, theanions in G3, although also interacting with the glyme molecules,exhibit one strong ion-ion interaction with a neighboring Li+ ion.These differences are consistent with the much higher aniontransport number for G4, where the anions are only weakly boundto their surroundings, and where one may expect the activationenergy for transport from one position to the next to be smallerthan in the case of G3, where the anion has to break away fromthe stronger interaction with its neighboring Li+. The anions in G4do possess continuous pathways for transport along the [101]direction.

In conclusion, we have shown that two small molecule solidelectrolytes, [CH3O(CH2CH2O)3CH3]:LiAsF6 and [CH3O(CH2-CH2O)4CH3]:LiAsF6, differing by only one ethylene oxide unitexhibit markedly different transport numbers,t+) 0.8 and 0.1,respectively. Such differences are related, on the one hand, to thepresence of tunnels for Li+ migration in the crystal structure of theG3 complex, but not G4, and, on the other hand, to the weakerbinding of AsF6

- in the structure of G4 than in G3. Such differencesare also consistent with the observed difference in the activationenergies for ion transport between the two complexes.

Acknowledgment. P.G.B. is indebted to the EPSRC and theEU for financial support.

Supporting Information Available: Experimental procedures andac impedance plot. This material is available free of charge via theInternet at http://pubs.acs.org.

References

(1) (a)Solid State Electrochemistry; Bruce, P. G., Ed.; Cambridge UniversityPress: Cambridge, 1995. (b) Kudo, T.; Fueki, K.Solid State Ionics;Kodansha Ltd.: Tokyo; VCH: Weinheim and New York, 1990. (c)Applications of ElectroactiVe Polymers; Scrosati, B., Ed.; Chapman &Hall: London, 1993. (d) Whittingham, M. S.; Huggins, R. A.J. Chem.Phys. 1971, 54, 414. (e) Christie, A. M.; Lilley, S. J.; Staunton, E.;Andreev, Y. G.; Bruce, P. G.Nature2005, 433, 50.

(2) Gadjourova, Z.; Andreev, Y. G.; Tunstall, D. P.; Bruce, P. G.Nature2001, 412, 520.

(3) (a) MacFarlane, D. R.; Huang, J.; Forsyth, M.Nature1999, 402, 792. (b)Alarco, P.-J.; Abu-Lebdeh, Y.; Abouimrane, A.; Armand, M.Nat. Mater.2004, 3, 476.

(4) Zhang, C.; Andreev, Y. G.; Bruce, P. G.Angew. Chem., Int. Ed.2007,46, 2848.

(5) (a) MacGlashan, G. S.; Andreev, Y. G.; Bruce, P. G.Nature1999, 398,792. (b) Stoeva, Z.; Martin-Litas, I.; Staunton, E.; Andreev, Y. G.; Bruce,P. G.J. Am. Chem. Soc.2003, 125, 4619.

(6) (a) Henderson, W. A.; Brooks, N. R.; Brennessel, W. W.; Young, V. C.,Jr. Chem. Mater.2003, 15, 4679. (b) Henderson, W. A.; Brooks, N. R.;Young, V. C., Jr.Chem. Mater.2003, 15, 4685.

(7) (a) Bruce, P. G.; Vincent, C. A.J. Electroanal. Chem.1987, 225, 1. (b)Evans, J.; Vincent, C. A.; Bruce, P. G.Polymer1987, 28, 2324.

JA073145F

Figure 3. The structure of G3:LiAsF6. Blue, lithium; white, arsenic; purple,fluorine; green, carbon; red, oxygen. Top: view of complete structure.Bottom: fragment of the structure showing one tunnel.

Figure 4. The structure of G4:LiAsF6. Top: the complete structure viewedalong the [101] direction. Bottom: one segment of the structure with thebdirection running from left to right.

C O M M U N I C A T I O N S

J. AM. CHEM. SOC. 9 VOL. 129, NO. 28, 2007 8701